1. Field of the Invention
This invention generally relates to an electronically controlled hydraulic braking system for a vehicle and, more particularly, to a braking system which is operable without a conventional master cylinder.
2. Description of the Related Art
Conventional hydraulic braking systems generally include a master cylinder operated in response to a pressure exerted on a brake pedal by a vehicle operator to generate a fluid pressure which is communicated to wheel cylinders to apply the brakes. In most braking systems, the pressure exerted by the operator is assisted by a brake booster such as a vacuum booster. For example, Japanese Unexamined Patent Publication No. 59-171745 discloses a hydraulic braking system having a master cylinder boosted by a vacuum booster. During anti-skid operation of the braking system, the fluid pressure communicated to the wheel cylinders is controlled by various solenoid-operated valves which, in turn, are controlled by an electronic control unit.
The use of a master cylinder has necessarily required relatively long hydraulic pressure lines to be provided between the master cylinder and the wheel cylinders, because the master cylinder is located far from wheel cylinders. This has been a bar to an improvement of the responsiveness of the braking system. Also, the provision for a brake booster has been an obstacle to the designing of a compact braking system. The use of various solenoid-operated valves for anti-skid and traction control operations has also complicated the design and installation of the system.